This invention relates to FM stereophonic broadcasting systems and, more particularly, an improved FM stereophonic broadcasting system which increases the broadcast coverage area over that of current biphonic service yet is compatible with existing monophonic and biphonic receivers.
The potential of FM sound broadcasting has long been recognized, and because of its relative immunity to electromagnetic interference and its ability to provide full audio bandwidth with low noise, was also selected as the transmission method for television sound. Although FM radio was hardly a universal success in the commercial sense when stereophonic broadcasts were first authorized in 1961, it was not long before the attraction of two-channel high-fidelity sound elevated FM to the status it enjoys today. However, although FM-stereo adds a new acoustical dimension to radio reception, it does so only at the expense of serious degradation of another high fidelity parameter, namely, the signal-to-noise ratio.
The noise penalty in stereophonic broadcasting is well known; less obvious, however, is the restrictive influence this phenomenon has on station coverage, which, for equivalent signal-to-noise ratio, typically may be only one fourth or one fifth the area of simple monophonic broadcasts. Several factors constribute to the higher noise levels and coverage losses resulting from multi-channel sound transmissions. When a broadcast station converts to biphonic service, monophonic coverage is reduced because signal power must be divided among the various components of the more complex baseband signal. (The term "biphonic" will be used hereinafter in order to clearly differentiate two-channel broadcasting from other forms of stereophony such as triphonic and quadraphonic broadcasting.) The biphonic signal-to-noise ratio is lower than monophonic signal-to-noise ratio because of the wide band width of the composite signal having the familiar equation EQU f(t)=M+p sin .omega./2 t+S sin .omega.t Eq. (1)
where M is the monophonic sum signal, p is the pilot, and S is the stereophonic difference signal. With a baseband spectrum extending to 53 kHz for biphonic transmissions, the noise level is particularly high because of the rising spectral characteristic due to frequency modulation. As shown in FIG. 1, the so-called "triangular" noise spectrum increases 6 dB per octave with increasing frequency of the composite-signal. Audio de-emphasis counteracts this somewhat as shown in FIG. 1, but the noise probem is still severe. After demodulation, the noise components of the difference channel subcarrier are added, statistically independent, to the noise already present in the monophonic signal during audio dematrixing.
Any precise computation of the theoretical loss of signal-to-noise ratio must taken into account factors such as the effect of de-emphasis, the format of the audio test signal (which is assumed for the computation), and interleaving. Interleaving is the interesting phenomenon whereby with certain audio signals the peak amplitude of the sum of the main channel signal and the sub-channel signal may be less than the sum of the peak amplitudes of these channels, thus permitting the interleaved signals to be raised to full modulation, with a resultant improvement in the signal-to-noise ratio. These factors have been studied by a number of researchers, and a calculation of the signal-to-noise degradation in biphonic broadcasting was published by N. Parker and D. W. Ruby in a 1962 paper entitled "Some notes on the calculation of the S/N Ratio For a FM System employing a double sideband AM multiplexer signal", IEEE Trans. Broadcast Television Receivers (International Convention Issue), vol. BTR-8, pp. 42-46, April 1962. The authors assumed the transmission of the peak monophonic power available, i.e., no modulation of the subcarrier (L-R=0); while their report of 23 dB degradation has received widespread acceptance, the figure is not entirely representative of typical programming. More recently, undr EIA auspices, the subject has been studied in greater detail by the National Quadraphonic Radio Committee (NQRC) and in its final report to the Federal Communications Commission, vol. II, Chapter 1, section 1.4, November 1975, by J. Gibson, et al, entitled, "Signal And Noise Levels In Proposed Multiplexed Systems For FM Broadcasting Of Quadraphonic Sound", reaffirmed the 23 dB penalty for a monophonic test signal, but also, by virtue of using a wide variety of audio test signals, demonstrated that a penalty of 26 dB is more representative of stereophonic programming with wide audio separation. For monophonic receivers, the NQRC data predict noise degradation of 1 dB to 7 dB, depending on the paticular type of test signal used.
Such losses of signal-to-noise ratio also cause a reduction in the effective area of coverage of a broadcast station; this effect for a representative set of transmission and reception conditions, based on NQRC data published Jan. 15, 1976 as a Supplement to the aforementioned report of the NQRC to the FCC entitled, "Illustrations to Relations Between Signal-To-Noise Ratio and Range in Existing FM Broadcast Services And Proposed Systems For FM Broadcasting of Quadraphonic Sound", is illustrated in FIG. 2. As a basis for this illustration, the NQRC used the FCC FM Engineering charts for the estimated field strength exceeded at 50% of the potential receiver locations for at least 50% of the time with a dipole receiving antenna height of 30 feet. The transmitter height was assumed to be 1,000 feet with a 10 kilowatt effective radiated power at 98 MHz, and the receiver was assumed to have a 10 dB noise figure. For reception at a signal-to-noise ratio of 50 dB, the limit of station coverage would extend to a radius of 128 miles when monophonic transmission only is employed. However, with biphonic transmission, two-channel reception extends only to a 60-mile radius, and monophonic reception is reduced to 100 miles. Although in reality station service areas are often limited by co-channel and adjacent-channel interference rather than by noise, FIG. 2 represents a useful comparison of the theoretical limits.
A potential solution of the noise penalty problem is the use of companding systems which achieve noise reduction by compressing the dynamic range of an audio program before transmission and expanding it to its original dynamic range at the receiver, the effect of which is illustrated in FIG. 3. The "original program" signal, with a wide dynamic range and a low noise level is represented at the left of the figure, and in the center the program is shown compressed to approximately one-half its original dynamic range for transmission purposes. During transmission, additional noise is introduced at a level below that of the compressed program, but at a level which would have intruded on the program had it not been compressed. Finally, the "expanded program" is shown at the right reconstituted to its original dynamic range and with the transmission noise simultaneously reduced to an unobtrusive level. Companding systems exhibiting these properties have achieved success in various audio applications, including tape and disc recording. For its potential application to broadcasting, tests were conducted in Sweden in the early 1960's utilizing a companding system in the S channel of FM-AM and FM-FM transmission systems; favorable results were reported for the FM-FM transmissions, although the system was never fully implemented. The results are briefly described in Vol. X, Report 300-3, "Stereophonic Broadcasting," of the XIIIth Plenary Assembly, International Radio Consulative Committee (CCIR), Geneva, 1974. Significant improvements in companding systems have been achieved during the last 20 years, and there is now renewed interest in the application of companding in broadcast systems as exemplified by the current examination by the Multichannel Sound Committee of the Broadcast Television Systems Committee of the potential application of companders to the S channel for television audio.
Given the recent advances in the art of audio companding, it is also appropriate to again examine its potential application to FM radio broadcasting. Currently, some broadcasters utilize Dolby-type encoding to provide modest noise reduction in receivers equipped with appropriate expanders and, relatively acceptable playback with simple receivers not having expanding capability. However, the requirement that compatibility with simple receivers must be maintained inhibits the potential for truly significant noise reduction in the other (expander-equipped) receivers.